The use of prosthetic implants for replacing or supplementing fractured, damaged, or degenerated skeletal bone in a mammalian body is commonplace in the medical arts. Usually, the prosthetic implant device is made of a biocompatible metal such as stainless steel, cobalt-chromium-molybdenum alloy, tungsten, titanium, cobalt-chromium-tungsten-nickel, and similar alloys. Most often, the prosthetic implant device is intended to become a permanent part of the skeletal structure.
One well-known technique for permanently attaching a metallic prosthesis to an adjoining bone or bones is to secure the implant in place with a polymethyl-methacrylate cement. By way of example, U.S. Pat. No. 5,360,446 describes a method of manufacturing prosthetic implant devices that may be secured into human bodies in this manner.
One of the problems that occur in implant devices is the gradual loosening of the prosthesis from the bone over time. This problem is especially prevalent where the prosthesis is subject to large functional loads and sheer stresses. Crania-facial implants, which are commonly used in the reconstruction or replacement of single teeth, are particularly prone to this problem. For instance, the difficulty in achieving a dental prosthesis that is strongly bonded to maxillary and mandibular bone, and which can withstand large sheer and tensile stress loads, has lead to the development of a variety of attachment mechanisms. Many of these mechanisms attempt to adaptively reform bone around the prosthesis, with the newly formed bone eventually bonding to the outer surface of the implant.
A variety of methods for promoting bone formation and attachment have been proposed. For example, U.S. Pat. No. 5,639,237 describes an endosseous dental implant having a dimpled surface texture for use in prosthetic reconstruction of crania-facial bones. The indented surface increases the surface area for bone proliferation, thereby enhancing the mechanical fixation/anchoring strength of the dental implant as compared to ordinary dental implants having a similar geometry.
Other approaches have attempted to strengthen the attachment of the bone at the site of the implantation. One such method is taught in U.S. Pat. No. 5,344,654, which claims that a strong bond is achieved between existing bone and the prosthesis by coating the prosthetic device with an osteogenic protein. To enhance endochondral bone formation, U.S. Pat. No. 5,656,450 teaches compositions and methods for effecting wound healing, specifically the activation of latent growth factor through matrix vesicles. Biodegradable polymeric implants are described which may be prepared containing latent growth factor, matrix vesicles, or matrix vesicle extract. An osteogenic device capable of inducing the formation of endochondral bone when implanted in the mammalian body is also disclosed in U.S. Pat. No. 5,645,591. This device includes an osteogenic protein dispersed within a porous matrix comprising a polymer of collagen and glycosaminoglycan.
Yet another approach for improving the strength and stability of a dental implant is discussed in U.S. Pat. No. 5,383,935. According to the teachings of this patent, a prosthetic device for implantation into skeletal bone generates current flow for calcium phosphate mineral formation between the implant and the surrounding bone. The formation of calcium phosphate minerals at the implant-bone interface is described as encouraging bone attachment to the implant, thereby providing stronger fixation of the implant into the skeletal structure.
An altogether different technique for enhancing bone density at the region of the implant is described in U.S. Pat. No. 5,344,457. This reference teaches effectively transferring loading stress from a dental implant to the surrounding bone through the use of an implant having a tapered body shape. Application of a vertical force on the tapered implant produces a sheer force component in addition to the normal force component acting on the surrounding bone.
Despite the plethora of prior art approaches for securing an implanted structure into mammalian bone, failures still occur. These failures are primarily due to the inability of the non-cortical bone to support the load of the implant. The reason why is because the new bone growth surrounding the implant surface is usually weaker, or more porous, as compared to the cortical areas of bone. Since internal areas of bone tend to be softer, more porous, and less dense than outer, circumferential regions of bone, implants into these areas are prone to failure due to movement and a lack of surrounding bone structure. This is particularly true in the case of dental implants into the maxilla and mandible.
Thus, there is a need in the medical and dental arts for improving the strength and integrity of the bone that surrounds and attaches to a prosthetic implant device. As will be seen, the present invention provides a method and a structure for increasing the load bearing strength of the bone surrounding an prosthesis. In addition, the present invention provides a novel way to augment both endo and exo bone formation for a variety of applications.